The present invention relates in general to an electromagnetic soleniod device having an armature mounted for movement in one direction relative to a pole portion of a field member upon application of current to a winding surrounding the field member and, in particular, to a solenoid device wherein the armature is moved in the one direction a distance which is substantially proportional to the current applied to the solenoid winding.
Solenoids have been widely used as a means to control various remotely located devices such as switches or valves. However, since the armature of a conventional solenoid is attracted to the pole with a force that increases sharply as the distance between the armature and the pole decreases, such solenoids are typically used to provide control functions wherein a device must be moved between only two positions such as, for example, on/off or open/closed. It has been difficult to utilize conventional solenoids to control displacement sensitive devices because of stability problems in maintaining such a device at an intermediate position.
U.S. Pat. No. 3,993,972 discloses an electromagnetic device having a pole construction adapted to control the effective reluctance of the magnetic circuit to reduce the tendency for the magnetic force exerted on the armature to rise steeply as the air gap between the pole and the armature decreases. In the device of this patent, an armature is spring biased away from a pole piece and is attractable toward the pole piece when current is supplied to a winding surrounding the pole piece. The pole piece includes a portion of reduced cross section which provides a restriction in the magnetic circuit such that, as the magnetizing force through the pole piece increases, the effective reluctance of the magnetic circuit increases. This results in the distance through which the armature moves against the action of the spring to be substantially proportional to the current flowing in the winding.